1. Field of the Invention
The present invention relates generally to reading multilevel signals from an optical disc and writing multilevel signals to an optical disc. Methods and apparatuses are described for processing signals that are eventually written to and read from an optical disc. These signals produce marks on the optical disc that may vary in both reflectivity and length. The system disclosed provides a method of encoding and decoding the data, correcting for errors, synchronizing the data, controlling the DC content, establishing and recovering a clock signal, establishing and recovering the envelope of the signal, and compensating for signal distortion.
2. Relationship to the Art
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. It should be noted that in this specification, the term multilevel is used to indicate greater than 2 levels. In a traditional optical recording system, reflectivity of the recording media is modulated between two states. The density of data recorded on an optical recording medium may be increased by modulating the reflectivity of the optical recording medium into more than two states.
One type of optical recording medium that appears to be particularly suitable for multilevel signal modulation is phase change optical material. When a phase change material is heated by a writing laser, the reflectivity of the phase change material may be changed. The change in reflectivity may be controlled by adjusting the amount of heating of the material and the rate at which the material cools. This process is described further in xe2x80x9cLaser-induced crystallization phenomena in GeTe-based alloys. I. Characterization on nucleation and growthxe2x80x9d (J. Appl. Phys. 78 (8), 15 OCT 1995. p. 4906) by J. H. Coombs, et. al. (hereinafter xe2x80x9cCoombsxe2x80x9d).
After a phase change optical disc has been written, the intensity of a beam of light reflected from the disc is measured so that the multilevel data written to the disc may be recovered. U.S. Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter xe2x80x9cKobayashixe2x80x9d) discloses a system for recovering multilevel data from such an optical disc. FIG. 1 is a block diagram illustrating the system disclosed in Kobayashi for recovering such data. Analog data read from a detector is input from a mark length detecting circuit 101 and a reflectivity detecting circuit 102. The outputs of these circuits are sent to an analog-to-digital (A/D) converter 103. The A/D converter 103 includes an n-value circuit which determines the value that the signal corresponds to by comparing the signal to predetermined reference voltages. Subsequently, the n-value signal is converted into a binary signal by binary circuit 405.
While this system discloses the concept of reading a multilevel signal and converting it into a digital signal in a basic sense, no method is disclosed of handling various imperfections in optically read multilevel signals that in fact tend to occur. For example, it is not clear how a clock is recovered for the purpose of precisely detecting mark lengths and no method is disclosed for handling problems that tend to occur in real systems such as amplitude modulation and DC offset of the optically detected signal and noise.
In a conventional two level optical data storage system, information is stored in the lengths of the marks and the spaces between them. So long as the edge of a mark can be detected with enough precision to distinguish between marks that differ in length by a minimum allowed amount, the system can operate reliably. This edge transition between one reflectivity state and another can be detected by setting a threshold value and determining the time when the signal crosses the threshold. Slow amplitude variations that might interfere with this edge detection are removed by AC coupling the photodetector signal before the threshold detection circuit. Mark and space lengths are measured by counting how many clock periods are between the edge transitions. The reader clock periods are synchronized to the mark/space edges, thus ensuring that there are an integral number of clock periods in each mark/space.
In contrast, in a multilevel recording system, the amplitude of the signal carries information. The reader interprets the data signal to determine the amplitude of the signal at certain times. Therefore, the reader clock must be synchronized to the data stream to ensure that the reader is interpreting the signal at the proper time. Because of the blurring effect of the optics in a reader, the transitions between the different levels do not create sharp edges. It is therefore difficult to synchronize the reader clock to the data stream. A method of precisely aligning a read data stream is needed. Further, a multilevel system is more sensitive to fluctuations in the overall envelope of the data signal. AC coupling alone is not adequate to enable a sufficiently precise determination of the different amplitude signals. Another problem encountered in a multilevel optical disc system is DC compensation.
In order for a multilevel optical read system to reliably record and recover data, a method of handling these sources of error in reading an optical signal is needed.
Accordingly, a system for writing and reading multilevel marks on an optical disc is disclosed. The system includes an error correction encoding and decoding system, modulation and demodulation system, DC control system, amplitude correction circuit, a clock recovery circuit, a write strategy system, a system to focus and track the laser spot on the surface of the disc, a system to rotate the disc, and an interface to a computer system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium that includes certain types of marks that enable reliable data storage and recovery. Several inventive embodiments of the present invention are described below.
In one embodiment, an information block in a multilevel optical data system is disclosed. The information block includes a preamble having a timing acquisition area, a preamble level calibration area, and an equalizer training area. The information block also has a data block and a postamble.
In one embodiment, an information block in a multilevel optical data system is disclosed. The information block has a preamble that includes a timing acquisition area, a preamble level calibration area and a preamble equalizer training area. The information block also includes a first data block, a midamble including a midamble level calibration area and a midamble equalizer training area. The information block further includes a second data block and a postamble.
In one embodiment, a multilevel pattern of marks written to an optical disc is disclosed. The multilevel pattern includes a preamble having a timing acquisition area, a preamble level calibration area and an equalizer training area. The multilevel pattern also has a data block and a postamble.
In one embodiment, a multilevel pattern of marks written to an optical disc is disclosed. The multilevel pattern has a preamble including a timing acquisition area, a preamble level calibration area and a preamble equalizer training area. The multilevel pattern also has a first data block and a midamble including a midamble level calibration area and a midamble equalizer training area. The multilevel pattern further includes a second data block and a postamble.
In one embodiment, a method is disclosed for writing an information block in a multilevel optical data system. The method has the steps of defining a preamble including the steps of providing a timing acquisition area, inserting a preamble level calibration area and adding an equalizer training area. The method also includes adding a data block and appending a postamble.
In one embodiment, a method of writing an information block in a multilevel optical data system is disclosed. The method includes the steps of defining a preamble having the steps of providing a timing acquisition area, inserting a preamble level calibration area and adding an equalizer training area. The method also includes adding a first data block, and defining a midamble having the steps of inserting a midamble level calibration area and adding a midamble equalizer training area. The method further includes adding a second data block and appending a postamble.
In one embodiment, a format for an address in pregroove (AIP) in a multilevel optical disc system is adapted to storing or retrieving data arranged in physical information blocks having physical information block addresses. The format has a lead-in area, data area and lead-out area. The format includes a plurality of AIP frames. An integer number of the AIP frames correspond to each of the physical information blocks and are arranged as AIP blocks. Each of the AIP frames contains an AIP address where the AIP address relates to said physical information block address by the equation INT(AIP address/integer number)=physical information block address. The format also includes a special information area contained in the AIP frames in the lead-in area.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.